Stabilized uranium or uranium-plutonium nitride fuel

ABSTRACT

A nuclear fuel composition which exhibits minimal reaction with stainless steel cladding at reactor service conditions is provided comprising a metal-containing uranium or uraniumplutonium nitride composition which forms, under reactor service conditions, a UXN2 or (U, Pu)XN2 compound as a separate stable phase where X is a metal selected from the group consisting of vanadium, chromium and niobium. These compounds are formed by reaction of fuel with excess nitrogen as the nitrogen is produced from fuel burnup in the reactor. Thus, the nitrogen equilibrium pressure is stabilized to a value below that which will react with the stainless steel cladding.

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Leitnaker et al. 1 1 Jan. 2, 1973 s41 STABILIZED URANIUM 0R URANIUM-3,126,323 3/1964 Leggett 6161 ..176/91 x PLUTONHUM I RIDE L 2,756,4897/l956 Morris ..l76/9l x [75 Inventors: James M. Leitnaker, Kingston,Prim y Examiner-Carl D. Quarforth ,37763 Karl Spear AssistantExaminer-G. G. Solyst State Ollege' [680] Attorney-Roland A. Anderson[73 I Assignee: The United States of America as represented by theUnited States [57] ABSTRACT Atomic Energy Commission A nuclear fuelcomposition which exhibits minimal 7 V reaction with stainless steelcladding at reactor service led: 1970 conditions is provided comprisinga metal-containing [21] Appl No: 67,452 uranium or uranium-plutoniumnitride composition which forms, under reactor serv1ce conditions, a UXNor (U, Pu)XN compound as a separate stable U-S. R, phase where X is ametal elected from the group con- Clt sisting of anadium hromium andniobiu n These Fleld of Search 252/301-1 compounds are formed byreaction of fuel with excess 75/205 nitrogen as the nitrogen is producedfrom fuel bumup in the reactor. Thus, the nitrogen equilibrium pressureReferences Cited is stabilized to a value below that which will reactwith UNITED STATES PATENTS the stalnless steel claddmg. 3,096,263 7/1963Kingston ..7s 20s x 7 3 D'awmg Fgures 3,211,664 10/1965 Endebrock....23/347 X 3,213,032 10/1965 Hammond ....23/347 X 3,510,434 5/1970Weber et al. ..23/344 X PAIENTEDJM 2193 3703.433 SHEET 1 OF 3 INVENTORS.James M. Leilnaker Karl E. Spear, II

PATENTEDJM 2 I973 SHEET 2 OF 3 INVENTORS. James M. Leilnaker Karl E.Spear, II

PATENTED 2 I975 3. 7 O8 ,4 3 3 SHEET 3 BF 3 I600 I200 800 600 I 14001000 700 I --logP (otm) 5 e 7 e 9 1o 11 12 Fig. 3

INVEN TORS. James M. Leilnaker Karl E. Spear, ll

STABILIZED URANIUM OR URANIUM- ILIJTONIUM NITRIDE FUEL BACKGROUND OF THEINVENTION The invention described herein was made in the course of, orunder, a contract with the United States Atomic Energy Commission. Itrelates generally to nitride nuclear reactor fuels and more particularlyto a high temperature uranium or uranium plutonium nitride fuel which isstabilized to nitrogen release at reactor service conditions, i.e.,exposure to a neutron flux and operating temperatures which are selectedfor the particular reactor design.

The excellent nuclear and physical properties-fissile density, thermalconductivity, and thermal expansion of uranium nitrides or uraniumplutonium nitrides makes them particularly attractive as nuclear fuels,such as for a fast breeder reactor. One problem of such fuels, however,is that excess nitrogen is produced during service. This excess nitrogencomes about both through the fissioning of uranium or plutonium and thereaction of the nitride fuel with oxygen which may diffuse into thesystem. Excess nitrogen is undesirable because it reacts with the fuelcladding causing embrittlement.

It is, therefore, an object of this invention to provide a uranium oruranium plutonium nitride composition which exhibits minimal reactionwith stainless steel cladding at service conditions.

SUMMARY OF THE INVENTION We have discovered that a UXN or (U, Pu)XNcompound, X being a metal selected from vanadium, chromium or niobium,could be formed as a separate stable phase which serves as a nitrogensink to accommodate excess nitrogen within the composition in uranium oruranium plutonium nitride fuels as the fuel is burned and thus stabilizethe nitrogen pressure in the system to a value which will precludereaction with stainless steel. This UXN; or (U, Pn)XN compound is formedby providing an amount of the aforementioned metal in a uranium oruranium plutonium nitride composition and reacting the compositionwithin a nuclear reactor under service conditions. In one embodiment ofthe invention applicants demonstrated the formation of avanadium-containing uranium nitride composition by reacting uraniummononitride with vanadium nitride at 1,450C and 580 torr nitrogenwhereby a novel ternary compound UVN was positively identified as aseparate phase. The value of nitrogen equilibrium pressure in the UN-UVN-V N region was found to be between 0.08 and torr at 1,450C which isbelow the nitrogen equilibrium pressure (0.1 torr) over the chromiumnitride chromium system at 1,450C.

In another embodiment of the invention applicants demonstrated theformation of a stabilized chromium containing uranium nitridecomposition by reacting uranium mononitride and chromium nitride powdersat l,600C and 400 torr nitrogen whereby the ternary compound UCrN wasidentified as a separate phase. The value of the nitrogen equilibriumpressure in the UN-UCrN -Cr region was found to be below thecorresponding nitrogen equilibrium pressure over the chromium nitridechromium system.

Owing to the minimal reaction of these stabilized fuels with stainlesssteel cladding, the fuels of this invention provide a distinct advancein the liquid metal fast breeder reactor technology.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a phase diagram of theternary system uranium, nitrogen and vanadium.

FIG. 2 is a phase diagram of the ternary system uraniurn, nitrogen andchromium.

FIG. 3 is a plot of nitrogen equilibrium pressure over various nitridemetal systems having the general reaction of MXNU+2 2 MxN 'lNzmDESCRIPTION OF THE PREFERRED EMBODIMENT The stabilized fuel compositionsof this invention may be prepared from conventional materials.Generally, any uranium or uranium plutonium compound and any suitablemetal (i.e., vanadium, chromium, niobium or molybdenum) compound whichis provided in an effective amount may be used as the startingmaterials, subject only to the condition that the compounds can bereacted under reactor service conditions to form a composition having anitrogen equilibrium pressure below that necessary to nitride anycomponent in stainless steel. By an effective amount it is meantsufficient metal to provide the nitrogen sink within the reactedcomposition.

One embodiment of the invention consists of blending the desiredquantity of uranium or uranium plutonium mononitride and vanadiumnitride powders into a mixture and pressing the mixture into pellets.The pellets are then sintered with suitable adjustment of the nitrogencontent, if necessary, and fabricated into a suitable fuelconfiguration.

In another embodiment uranium (or uranium plutonium) dioxide is mixedwith finely divided carbon and an oxide of the desired metal, such asvanadium, and the mixture heated in nitrogen to form a nitrided mixtureof uranium (or uranium plutonium) and vanadium. The resulting nitridedmixture is then pressed into pellets, sintered with suitable adjustmentof the nitrogen content, if necessary, and fabricated into a suitablefuel configuration.

It should be apparent that the amount of metal will vary for each fuelcomposition. There is, however, a critical amount of metal for eachcomposition which, once it has been empirically determined, will operatesuccessfully in providing the requisite nitrogen sink, therebyaccommodating within the reacted composition excess nitrogen as it isproduced and precluding continued rise in the nitrogen equilibriumpressure to a value which will nitride the cladding.

In the vanadium containing uranium nitride fuel the quantity of vanadiumadded should be sufficient that the composition remains in the regionsbelow the UN- UVN, and UVN -V N tie lines (FIG. 1) during the life ofthe reactor fuel. Suitable vanadium concentrations are from about 4 atompercent to about 10 atom percent (based on 2 year life of a fast breederreactor fuel), the latter being an upper limit imposed by undesirabledilution of the uranium. Referring to FIG. 1, it may be seen that anumber of constant pressure )three-phase regions appear to exist in theU-V-N system; namely, UN-U-V, UN-V N-V, UN-UVN -V N, UN-UVN -U N;,, andU N -UVN -vN. While the last two regions are of constant pressure, thenitrogen equilibrium pressure at 1,400C was found to be sufficientlylarge, i.e., value was above the Cr N-C line shown in FIG. 3, to reactwith the chromium in the stainless steel cladding at anticipated reactorcladding temperature of about 800C. The composition, therefore, shouldalways be maintained below the UN-UVN and UVN -v N tie lines of FIG. 1;thus, providing a nitrogen pressure 0.08 atmosphere at 1,400C) whichwill not react with the stainless steel cladding at anticipated reactorcladding temperatures.

In the chromium containing uranium nitride fuel the quantity of chromiumadded should be sufficient that the composition is maintained below theUN- UCrN and UCrN -Cr tie lines of FIG. 2. Suitable chromiumconcentrations are from about 4 atom percent to about atom percent.

In a similar fashion to the vanadium and chromium containing fuelcompositions a niobium containing fuel composition may be formed underreactor service conditions to provide a fuel composition which isstabilized with respect to nitrogen release during fuel burnup.

While the exact phases present in the stabilized nitride compositions ofthis invention have not been conclusively established at temperature,the separate UXN or (U, Pu)XN compound, when formed in accordance withthis invention, acts, irrespective of the phase makeup of thesestabilized nitride compositions, as a buffer or sink for disposing ofexcess nitrogen as it becomes available by preventing the nitrogenequilibrium pressure over the system from rising to a value whereinreaction with the most easily nitrided cornponent of stainless steelcladding would take place.

The temperature at which the separate UXN or (U,

Iu)Xl I compound is formed may vary over a wide range. It is importantinsofar as the temperature parameter is concerned that a high enoughtemperature be used to insure that the nitrogen sink can be formedthereby achieving stabilization. Temperatures such as to be observed atthe clad-fuel interface under reactor service conditions, i.e., about800C are quite suitable for the reaction which achieves stabilization.

EXAMPLE I Ternary samples containing uranium, vanadium and nitrogen wereprepared using mixtures of uranium mononitride and vanadium nitride,nitrided uranium vanadium alloys, or mixtures of uranium mononitride andvanadium and formed into 0.25-inch diameter pellets compacted from 325mesh (less than 44 ,um) powders.

The respective samples which were contained in tungsten crucibles wereheated in'a vacuum induction furnace under constant pressure andtemperatures for various lengths of time and then cooled to below red'heat in 3 to 5 minutes. Temperatures and nitrogen pressures used were1,400 to 2,000C and to 650 torr; a few samples were heated in vacuum.The nitrogen gas was passed through Drierite, a special form ofanhydrous calcium sulfate commercially available from the W. A. HammondDrierite Co., Xenia, Ohio, and over niobium 1 percent zirconium chips at800C be fore use.

The phases present in each sample were determined by X-ray powderphotographs and the phase diagram (FIG. 1) deduced from the resultsgiven in Table 1 below. The X-ray samples were prepared in anargonfilled glove box. Lattice parameters were determined with the useof Debye-Scherrer X-ray films by well known techniques, employing eithernickel-filtered Cu K01 or vanadium-filtered Cr Ka radiation (a a and awere 1.54051, 1.54433 and 1.54178 A, respectively for copper and2.78962, 2.29351 and 2.29092 A for chromium.

TABLE I Uranium Annealing conditions to vanadium Temper- X-ray results aStarting molar ature P (N2) b Time Sample material ratio C.) (torr)(hr.) VzN VN UVNz UN UzNa U02 1 UN,VN 0.126 1,450 590 2 UN,VN 0.2361,450 580 3 UN, VN 0. 258 1, 600 400 41 0. 279 1, 800 200 42 0. 279 1,600 2X10- 43 0.279 S2X10' 44 0. 279 1, 700 52 45 0. 279 1, 700- 100 460. 279 1, 785 400 47 0. 279 2, 000 400 48 0.279 1, 600 200 49 0. 2791,609 100 5 0. 490 1, 600 400 61"- 0.501 1, 795 203 62- 0. 501 1, 700407 63-.-" 0.501 1, 550 2X10" 71 0. 54 1,410 590 72. 0. 54 1,600 195 730. 54 1, 600 81 0. 93 1, 400 52x10- 82 0.93 1,600 2X1CI- 91 1.03 1,790203 92. 1.03 1, 700 406 93 1.03 1, 550 2 10 10 1.19 1,450 590 111 1. 1,790 207 112 1. 95 1, 690 407 12 6. 04 1, 450 590 141-. 7. 3 1 450 570142 7. 3 1, 600 195 143 142 7. 3 1, 600 15... UN,VN 10.5 1,450 650 eNumbers refer to samples, Whose residues were used as starting material.

\ b All samples were cooled in the atmosphere shown except samples44-48. The nitrogen atmosphere maintained during the heating of thesefive samples was pumped our as the samples cooled.

s Relative intensities of each d A uranium-vanadium alloy was nitrldod.

phase diffraction pattern are indicated by s=strong, m=medium, w=weak,v=very.

" The temperature was varied between 1,1100 and 2,000 C. for 20 minutes.1 Three lines of an unknown pattern were observed in samples 44-48.

From FIG. 3 the nitrogen equilibrium pressure at 1,400C over thechromium nitride chromium (chromium being the most easily nitridedcomponent of stainless steel) is 0.1 atmosphere. The nitrogenequilibrium pressure at 1,400C for the composition with the area UN-UVNV N was determined to be between 0.08 and atmosphere. At 800C, which isthe anticipated cladding temperature for stainless steel clad liquidmetal fast breeder fuels, the nitrogen equilibrium pressure over thechromium nitride chromium system is 10" atmosphere and for thecomposition within the area UN-UVN -V N would be below 10 atmosphere.Accordingly, it may be seen that the compositions are stabilized withrespect to nitrogen release and would preclude the nitrogen fromreacting with the stainless steel cladding.

While most of the X-ray films of the ternary samples showed weakdiffraction lines due to U0 the solubility of oxygen in UN, UVN VN and VN is small when the possibility for forming U0 exists; and it isestimated that the amount of oxygen dissolved in these phases is lessthan 0.2 weight percent. For compositions within the area UN-UVN -V Nthis small amount of oxygen would not appreciably affect the phaserelationships.

A novel ternary compound UVN which is stable as a separate phase withrespect to decomposition into UN and VN up to at least 2,000C, wasestablished. While the lattice parameter of the ternary compound was notmeasured as functions of the total content of uranium, vanadium, ornitrogen in the samples, visual comparisons of UVN; lines in films ofall samples containing this phase showed no noticeable shift in theselines. From these comparisons it may be concluded that the homogeneityrange of UVN is probably small. The ternary UVN compound is isomorphouswith UMoC has a calculated enthalpy of reaction of l29 8 kcal/mole at298K and has an entropy at 298K of about 23.9 e.u. The orthorhombiclattice parameters of the ternary UVN compound were calculated to be a,,5.451 i 0.002, b,,= 3.185 1 0.001, and c 10.667 2*: 0.003 A, theuncertainties being as computed standard deviations. X-ray data fororthorhombic UVN is given in Table 11 below.

TABLE 11 d-Spacing, A Relative Intensity h k l Observed CalculatedObserved Calculated 0 1 4.79 4.85 w 4.4 0 2 3.77 3.81 w 2.2 I 1 3.033.05 mw 3.0 0 3 2.958 2.978 m 4.3 O 0 2.716 2.726 w 1.0 0 4 2.651 2.667ms 2.9 I 1 2.662 1.4 0 1 2.629 2.641 m 3.1

l 2 2.434 2.444 vs 10.0 0 4 b 2.396 vw 0.3 1 3 2.372 0.4 1 3 2.171 2.175mw 1.8 0 3 2.163 1.2 1 0 2.067 2.071 m 1.5 l 1 2.031 2.033 w 0.6 1 21.930 0.4 1 4 1.905 1.914 w 0.3 0 4 1.906 0.9 0 6 1.770 1.778 ms 0.2 l 51.773 1.7 0 2 1.719 1.720 mw 1.0 0 6 1.690 0.2 1 5 1.676 1.686 mw 0.2 05 1.680 1.2 1 4 1.632 1.636 mw 1.0 2 0 1.590 1.593 mw 1.1 l 1 1.5581.561 ms 1.8 2 1 1.509 1.513 vw 0.4 0 4 1.498 1.502 vw 0.3

1 1 6 1.491 1.493 m 1.6 2 l 5 1.486 0.6 1 2 2 1.465 1.469 mw 0.4 1 0 71.468 1.0 3 l 3 1.441 1.443 vs 2.0 1 2 3 1.403 1.404 ms 1.2 2 2 0 b1.375 vw 0.4 0 l 7 1.375 0.1 0 2 4 1.367 1.2 2 2 1 1.365 1.364 s 1.3 4 00 1.363 0.2 4 0 1 1.349 1.352 m 0.8 2 l 6 1.332 1.349 mw 0.9 0 0 8 1.3330.4 1 l 7 b 1.333 vw 0.4 1 2 4 1.326 0.2 1 0 8 b 1.295 vw 0.3 2 2 31.282 1.282 w 0.9 4 0 3 1.273 0.4 3 0 6 1.270 1.271 mw 0.6 3 l 5 1.2690.4 4 1 0 1.254 1.253 m 0.7 4 1 1 b 1.244 vw 0.2 2 2 4 1.223 1.222 s 1.34 l 2 1.220 0.2 4 0 4 b 1.214 vw 0.5 1 l 8 1.200 1.200 m: 1.1 2 O 8 l198 0.2 0 2 6 1.186 1.186 vw 0.5 4 1 3 1.182 0.1 3 2 2 1.168 1.169 vs3.9 l 2 6 1.159 1.0 l 0 9 1.156 1.158 .r 0.4 2 2 5 1.156 7.2 4 0 5 b1.148 m 6.0

The calculated values are for lattice parameters a 5.451, b, 3. 185, and0 10.667 A.

"A very weak line was observed, but its d-spacing was not measured. lheline was very diffuse.

EXAMPLE 11 Ternary samples containing uranium, chromium, and nitrogenwere prepared using mixtures of uranium mononitride and either chromiumor chromium nitride powders 44 pm) and pressed into 0.25-inch diameterpellets. The pellets were heated at constant temperature and pressurefor various lengths of time in tungsten crucibles in an inductionfurnace as in Example 1 and were cooled to below red heat in 3 to 5minutes. Patterns of the starting materials showed only the lines of therespective phases; no impurity lines were observed.

The phases present in each sample were determined by X-ray powderphotographs which were taken with Cu Ka radiation and the phase diagram(FIG. 2) deduced from the results given in Table 111 below.

The molarratios were calculated from the mams of starting material.Samples 1, 2, and 3 were cooled in 400 torr N samples 4, 5, and 6 hadthe nitrogen pumped out while they cooled. To cool below red heatrequired 3 to 5 min.

The relative intensities are indicated by .r =strong, m medium, and wweak.

Sample 4 was annealed at 1600C and then 1300C in the same experiment.

Ductile metallic pieces were observed during preparation of x-rayspecimens.

From the fact that the ternary compound UCrN can exist in equilibriumwith the metal demonstrates that the nitrogen equilibrium pressure inthe area UN- UCrN -Cr is lower than that for Cr N-Cr line shown in F IG.3 and therefore, the composition is stabilized with respect to nitrogenrelease. All of the samples had uranium-to-chromium molar ratios of 1.0or smaller. A ternary compound approximating UCrN was established. Thed-spacings and line intensities'of the ternary compound UCrN are givenin Table IV below.

TABLE IV "s= strong, m =medium, w weak, v very.

What is claimed is:

1. A nuclear fuel comprising a uranium or uranium plutonium nitridecomposition containing a UXN or (U, Pu)XN compound as a separate stablephase wherein said composition has, at a temperature in the range5001,400C, a nitrogen equilibrium pressure below that of the Cr N-Crline of FIG. 3 and wherein X is a metal selected from the groupconsisting of vanadium, chromium, and niobium.

2. The fuel of claim 1 wherein said compound comprises UVN 3. The fuelof claim 1 wherein said metal-containing uranium or uranium plutoniumnitride composition lies within the three phase UN-UVN -V N regiondisplayed in FIG. 1, said composition having a nitrogen equilibriumpressure at 1,400C of less than 0.08 atmosphere.

4. The fuel of claim 1 wherein said compound comprises UCrN 5. Anitrided nuclear fuel composition of the basic formula UXN or (U, Pu)XNwherein X is essentially of a metal selected from the group consistingessentially of a metal selected from the group consisting essentially ofvanadium, chromium, and niobium, the concentration of the aforementionedelements in said composition being such that, at a temperature in theran e 500l ,400 C. it will not nitride stainless steel.

. The composition of claim 5 in Wl'llCl'l the selected metal exists as anitride and contains a separate UXN compound where the compositioncontains uranium and contains a separate (U, Pu)XN compound where thecomposition contains plutonium.

7. The composition of claim 6 wherein X is vanadi-

2. The fuel of claim 1 wherein said compound comprises UVN2.
 3. The fuelof claim 1 wherein said metal-containing uranium or uranium - plutoniumnitride composition lies within the three phase UN-UVN2-V2N regiondisplayed in FIG. 1, said composition having a nitrogen equilibriumpressure at 1,400*C of less than 0.08 atmosphere.
 4. The fuel of claim 1wherein said compound comprises UCrN2.
 5. A nitrided nuclear fuelcomposition of the basic formula UXN2 or (U, Pu)XN2 wherein X isessentially of a metal selected from the group consisting essentially ofa metal selected from the group consisting essentially of vanadium,chromium, and niobium, the concentration of the aforementioned elementsin said composition being such that, at a temperature in the range500*-1,400*C. it will not nitride stainless steel.
 6. The composition ofclaim 5 in which the selected metal exists as a nitride and contains aseparate UXN2 compound where the composition contains uranium andcontains a separate (U, Pu)XN2 compound where the composition containsplutonium.
 7. The composition of claim 6 wherein X is vanadium.